A. Field of Art
The present invention relates to the field of wavelength division multiplexed (WDM) optical communication networks, and more particularly to the architecture and operation of wavelength selective switches (WSS) and related devices to minimize bandwidth-narrowing penalties in such networks.
B. Description of Related Art
Modern optical communication networks often employ wavelength-multiplexed optical signals in a single transmission optical fiber to increase the transmitted bandwidth. Such signals are typically deployed on a pre-defined frequency grid, such as the standard grid defined in ITU standard G.709. Each signal transmitted at one of these standard frequencies or wavelengths propagates throughout the network in its own distinct “channel” of that fiber. With such a grid, the center frequencies of adjacent channels are typically spaced at regular intervals, such as 50 GHz or 100 GHz. Alternatively, the center frequencies may be at arbitrary intervals, thereby forming an adjustable frequency grid. In these systems, a wavelength-division multiplexer is used to combine a plurality of signals onto a single transmission fiber, with the frequency of each signal having a different nominal grid frequency, and a wavelength-division demultiplexer used to separate the signals at the end of the transmission fiber so that each signal is directed to a distinct optical receiver. Each WDM signal is thereby capable of carrying separate and independent client traffic.
Optical networks may be configured in various topologies, such as point-to-point, ring, linear bus, or mesh. The topology employed in a particular network is determined by the interconnections among the nodes and available fiber in that network. WDM networks may be deployed with fixed add/drop multiplexers, colorless add and drop couplers, and/or reconfigurable optical add/drop multiplexers (ROADMS). A ROADM at a network node may be constructed using one or more wavelength-selective switches (WSS) configured to selectively add, drop, or block channels based on their grid frequency.
WSS technology is available today that supports more than 80 channels through a single device typically having from 3 to 10 input/output ports. However, the optical technology can be extended to higher channel counts and port counts. Several types of WSS optical modules have been proposed (see, e.g. U.S. Pat. Nos. 7,492,986 and 6,487,334).
Regardless of the particular technology employed, a WSS typically has the ability to selectively direct a signal from an input port to any output port based on the frequency (or wavelength) of the signal. The route or path of a signal originating at a source node of the network, and passing through one or more intermediate nodes before reaching a destination node, may be deemed to include its path within a node as well (i.e., between one or more WSS input ports and one or more WSS output ports).
A ROADM node may also have: (1) a channel monitor that monitors the power at each frequency grid point; and, (2) a means of attenuating the power of each channel transmitted in a fiber. The channel monitor and power adjustment may be integrated into the WSS module or implemented as separate modules. Regardless of the particular implementation, the combination of a channel monitor with power control enables the functions of (1) balancing the channels at one or more points in the ROADM node and (2) selectively blocking channels by maximizing their attenuation.
WSS technology, coupled with a management overhead channel, enables remote network reconfiguration from a central network operations center (NOC). The management channel can be transmitted over an external IP network, a dedicated optical service channel, or within the embedded overhead of an optical signal.
In a typical deployment, before any channels have been added to the network, all channels of the WSS are set at full attenuation, which can be referred to as the blocking state or “B” state. This prevents amplified spontaneous emission (ASE) from optical amplifiers from propagating and being amplified through the network when a particular channel is not present in the network. Circulating ASE is of particular concern in networks with a closed optical path, such as ring topologies, because of the optical power instability it can cause. In an amplified network with one or more closed paths (such as in a ring network), each grid channel is typically blocked or dropped at least once to prevent ASE instability.
If a channel is being reused, i.e. the same channel frequency is being reused by two or more non-overlapping separate signals, then the light from the first signal must be effectively blocked before the second signal is added so as to prevent cross-talk penalties. Dropped signals are not blocked in broadcast applications, however, because that same signal must propagate to the other nodes receiving the broadcast signal.
A WDM network is typically deployed with a “guard-band” between the nominal frequencies. A guard band is required because: (1) practical optical filters used in WSS modules have a finite slope between their pass bands and stop bands; (2) optical signals have a modulation bandwidth on the order of their bit rate; and, (3) errors occur in laser frequencies and center frequencies in optical filters due to manufacturing tolerances, calibration errors, temperature drifts, and component aging. For example, 100 GHz channel spacing may be used for channels at 10 Gb/s or 40 Gb/s, which have full-width at half-maximum bandwidth less than 50 GHz. As optical networks have evolved, the maximum bit rate has increased, with 100 Gb/s networks currently being deployed, with a reduced grid frequency spacing of 50 GHz. Thus, the relative guard band is decreasing over time while requirements on frequency accuracy are increasing.
A significant design issue for WSS filters is the problem of bandwidth narrowing. As client signals traverse WSS modules in a network where each WSS is set to attenuate unused adjacent channels (e.g., unused channels at 193.9 THz and 194.1 THz adjacent to signal channel 194.0 THz), the effective passband of the WSS cascade is reduced, which can lead to bit errors. For an optical signal in a particular channel propagating through a WSS network, a bandwidth narrowing event occurs at each WSS where one or both of the channels adjacent to the signal are set to a different physical state (e.g., “pass through” as compared to “blocking” or “add”) than the state of the channel of the given signal.
All optical filters have a useable passband which is less than that of an ideal filter due to the finite slope of a manufacturable filter passband. Moreover, the useable bandwidth of cascaded filters decreases as more filters are inserted in the signal path. This bandwidth narrowing effect has led WSS designers and manufacturers to increase the effective Gaussian order of the WSS pass band spectral shape [See for example “Wavelength-Selective Switches for ROADM Applications” in IEEE Journal of Selected Topics in Quantum Electronics, vol 16, pp. 1150-1157, 2010]. Such techniques have improved, but not eliminated, the problem of bandwidth narrowing. Therefore, as the bit rate (and hence bandwidth) of optical signals increases, and the size of ROADM networks increase, there remains a need for more effective techniques of minimizing WSS bandwidth narrowing.
Accordingly, a solution is desired that provisions channels carrying client signals in WSS modules so as to minimize bandwidth narrowing while still preventing significant ASE circulation and coherent cross-talk among different transmitters operating at the same frequencies.